Diversity

The family Eleotridae contains about 35 genera and 150 species, and of these most are found in the tropical Indo-Pacific. They are commonly known as sleepers, or gudgeons in Australia and New Guinea, and in New Zealand they are called bullies. The family is similar to Gobiidae but generally lacks the pelvic fin fusion that creates a “sucking disc” in gobies. The majority of eleotrids lives in brackish or fresh water. Only a few species are truly marine, but many fresh water species have a marine larval stage and return inland as juveniles. They are carnivorous, and in turn are eaten by humans in many parts of their range. Sixteen species of eleotrids are listed as near threatened or vulnerable to extinction. (Berra, 2001; Helfman, et al., 1997; Moyle and Cech, 2000; Nelson, 1994; The World Conservation Union, 2002; Wheeler, 1985)

Geographic Range

Eleotrids can be found worldwide between the 40th parallels (tropical and subtropical regions), reaching farther south in New Zealand. They occur on five continents and are common in the islands of the Indo-Pacific. (Allen and Robertson, 1994; Berra, 2001; Nelson, 1994)

Habitat

Eleotrids occupy mostly fresh and brackish water. While only a few species are marine, many freshwater eleotrids spend their larval stage in the ocean and return inland as juveniles. They live in tropical and subtropical areas, and a few occur in warm temperate waters. They are common residents of mangrove and other estuarine environments. Eleotrids can be found near rocky reefs in bays, in intertidal areas, and in streams or ponds. Some prefer still water among aquatic vegetation. Many live on muddy substrates (bottoms)—most eleotrids are benthic (bottom-dwelling)—but a few are free-swimming. (Allen and Robertson, 1994; Berra, 2001; Graham, 1997; Kuiter, 1993; Nelson, 1994; Wheeler, 1985)

Physical Description

Eleotrids are small, most measuring between three and 20 cm, although one species, Oxyeleotris marmorata, reaches 66 cm. They are similar to members of the family Gobiidae, with similar head shape, an elongate body, no lateral line, and two separate dorsal fins. They are generally distinguished from Gobiidae on the basis of their separated pelvic fins that do not form a sucking disc. The degree of pelvic fin separation varies, however, and cannot reliably be the only characteristic used to identify eleotrids. Eleotrids can have cycloid or ctenoid (rough-edged)
scales. They lack sensory pores, and have canals only on the head. Their mouths, filled with several rows of conical teeth, can be upturned or terminal, but never inferior. The first dorsal fin contains two to eight flexible spines, and a single spine heads the second. Many eleotrids have a well-developed swim bladder, although they are generally benthic (bottom-dwelling). Some have dull, brownish or dark coloration, while others are colorful. One species that lives in wells and sinkholes, Milyeringa veritas, is white or pinkish and has no eyes. Some eleotrids may be permanently sexually monomorphic (males and females alike), as is the case with most reef-dwelling gobies, but males of some species develop distinctive coloring for courtship, or when excited by the presence of a competitive male. During the breeding season a hump on the head behind the eyes appears on males in the species Hypseleotris galii. (Click here to see a
fish diagram). (Allen and Robertson, 1994; Berra, 2001; Graham, 1997; Helfman, et al., 1997; Kuiter, 1993; Nelson, 1994; Thresher, 1984)

Development

Many freshwater eleotrids are amphidromous: after hatching they float downstream to brackish or marine waters where they pass through a planktonic larval stage, growing and feeding for a few months before they migrate back to fresh water as juveniles. This marine stage is thought to indicate that Eleotridae originated as a marine family. Some freshwater gobies develop without a planktonic larval stage, becoming a benthic juvenile directly after hatching, and this may be the case for some eleotrids as well. (Berra, 2001; Helfman, et al., 1997; Moyle and Cech, 2000; Nelson, 1994; Thresher, 1984)

Reproduction

No information was found that addresses mating systems specifically in eleotrids, but Thresher (1984) includes Eleotridae in his general account of reproduction in the suborder Gobioidei. Gobies exhibit a wide variety of mating systems but most seem to be promiscuous, either organized into a hierarchical social system or small territories maintained by individuals. At least one species of eleotrid (Thalasseleotris adela) is usually found in pairs. In gobies, a typical mating sequence begins with nest preparation by the male, which involves clearing and cleaning the area where eggs will be deposited. The female’s readiness for spawning is evidenced by her swollen ventral area. The male swims back and forth between the female and the nest site and in some cases he will nudge her with his snout. Eleotrid courtship behavior probably follows a similar pattern, with some males assuming intense courtship colors and leading females to the nest. (Berra, 2001; Kuiter, 1993; Thresher, 1984)

Eleotrids attach their eggs to vegetation or a substrate (bottom surface). Females in Hypseleotris compressa deposit up to 3000 eggs. No other information was found that addresses reproduction specifically in eleotrids, but Thresher (1984) includes Eleotridae in his general account of reproduction in the suborder Gobioidei. Most gobies have extended spawning seasons with peak spawning depending on the species, but in colder regions breeding may only occur once or twice a year. Females may deposit from five to several hundred eggs, which the male then fertilizes. In estuarine species the lunar cycle is thought to play a role in spawning behavior as well as larval recruitment. (Berra, 2001; Thresher, 1984)

Males in Hypseleotris compressa guard the nest. No other information was found that addresses parental care specifically in eleotrids, but Thresher (1984) includes Eleotridae in his general account of reproduction in the suborder Gobioidei. In most cases, male gobies guard the eggs after they are fertilized, and even if females are permanently paired they rarely take part in parental care. The young probably stay close to adults for a period of time after hatching. In some freshwater island species parental care is not practiced at all. Many eleotrids may fall into this category, since the larvae are carried downstream to the ocean where they feed and grow before ascending the freshwater streams. (Berra, 2001; Böhlke and Chaplin, 1968; Thresher, 1984)

Lifespan/Longevity

No information was found regarding the lifespan in Eleotridae. However, some small gobies (similar in many respects to eleotrids) may mature quickly and live only one or two years. (Thresher, 1984)

Behavior

Eleotrids earned their common name, “sleeper,” for their habit of remaining still on the bottom or positioned near an object in the water while they wait for prey. When chasing prey, however, they can perform very quick movements. Some, such as Hypseleotris klunzingeri, form dense schools in midwater. Many eleotrids occupy habitats that can become low in oxygen (hypoxic), and some can breathe air through the skin on the front of the head. When Dormitator latifrons encounters hypoxic waters, oxygen begins to fill the gas bladder and the top surface of the head begins to fill with a thick bed of capillaries. In order to attain positive buoyancy and keep the head exposed to air, the gas bladder must fill completely, but this may take up to six hours. Until the gas bladder fully inflates, these fish use submerged objects to prop themselves up. The top of the head does not become fully vascularized for 10 to 20 days. (Berra, 2001; Graham, 1997)

Ecosystem Roles

Often the major predators in island stream systems, eleotrids form an important element of the freshwater fauna in the regions in which they are found. Many impact not only the crustaceans, fishes, and insects on which they feed as adults, but also join the marine planktonic ecosystem as larvae. Eleotrids are able to occupy various habitats, including brackish and hypoxic (low-oxygen) areas. (Allen and Robertson, 1994; Berra, 2001; Graham, 1997; Helfman, et al., 1997)

Economic Importance for Humans: Positive

Eleotrids are used for food in many regions. The freshwater species Dormitator maculates (fat sleeper) is considered a delicacy in Thailand, Sumatra, Borneo, and the Malay Peninsula. Eleotrids are easy to breed in captivity and are used as aquarium fish. (Berra, 2001; Wheeler, 1985)

Economic Importance for Humans: Negative

No specific information was found concerning any negative impacts to humans.

Conservation Status

Based on information gathered in 1994, 16 species within Eleotridae are near threatened or vulnerable. Their populations are either small in terms of adult individuals or in terms of total area in which they are found, rendering them vulnerable to human exploitation, pollution, hybridization, competitors, parasites, or disease. (The World Conservation Union, 2002)

Contributors

Glossary

Australian

Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

Ethiopian

living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

Neotropical

living in the southern part of the New World. In other words, Central and South America.

Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

Palearctic

living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

benthic

Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

brackish water

areas with salty water, usually in coastal marshes and estuaries.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

coastal

the nearshore aquatic habitats near a coast, or shoreline.

cryptic

having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

estuarine

an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.

external fertilization

fertilization takes place outside the female's body

fertilization

union of egg and spermatozoan

food

A substance that provides both nutrients and energy to a living thing.

freshwater

mainly lives in water that is not salty.

insectivore

An animal that eats mainly insects or spiders.

intertidal or littoral

the area of shoreline influenced mainly by the tides, between the highest and lowest reaches of the tide. An aquatic habitat.

island endemic

animals that live only on an island or set of islands.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

male parental care

parental care is carried out by males

monogamous

Having one mate at a time.

motile

having the capacity to move from one place to another.

natatorial

specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.

oceanic islands

islands that are not part of continental shelf areas, they are not, and have never been, connected to a continental land mass, most typically these are volcanic islands.

oriental

found in the oriental region of the world. In other words, India and southeast Asia.

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

pet trade

the business of buying and selling animals for people to keep in their homes as pets.

piscivore

an animal that mainly eats fish

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

polygynous

having more than one female as a mate at one time

reef

structure produced by the calcium carbonate skeletons of coral polyps (Class Anthozoa). Coral reefs are found in warm, shallow oceans with low nutrient availability. They form the basis for rich communities of other invertebrates, plants, fish, and protists. The polyps live only on the reef surface. Because they depend on symbiotic photosynthetic algae, zooxanthellae, they cannot live where light does not penetrate.

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

seasonal breeding

breeding is confined to a particular season

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

sexual ornamentation

one of the sexes (usually males) has special physical structures used in courting the other sex or fighting the same sex. For example: antlers, elongated tails, special spurs.

social

associates with others of its species; forms social groups.

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

Disclaimer:
The Animal Diversity Web is an educational resource written largely by and for college students. ADW doesn't cover all species in the world, nor does it include all the latest scientific information about organisms we describe. Though we edit our accounts for accuracy, we cannot guarantee all information in those accounts. While ADW staff and contributors provide references to books and websites that we believe are reputable, we cannot necessarily endorse the contents of references beyond our control.

This material is based upon work supported by the
National Science Foundation
Grants DRL 0089283, DRL 0628151, DUE 0633095, DRL 0918590, and DUE 1122742. Additional support has come from the Marisla Foundation, UM College of Literature, Science, and the Arts, Museum of Zoology, and Information and Technology Services.